The transduction of a capacitive microphone is based on modulation of the electrical capacitance between a diaphragm and a backplate. With the capacitance biased by means of a constant charge, the voltage change due to the modulation can be read out by means of a buffer amplifier.
In MEMS capacitive microphones, the constant charge is approximated by connecting a DC bias voltage across the capacitor with a high impedance in series. In combination with the sensor capacitance, the high impedance forms a low-pass filter, so that for frequencies above the filter cut-off the constant charge condition is fulfilled.
A state-of-the-art implementation of the high impedance is a pair of antiparallel diodes, or a CMOS circuit (see e.g. U.S. Pat. No. 7,149,317 or U.S. Pat. No. 7,221,766). For small signals, the impedance of such pair can be very high. For large voltage swings however the impedance rapidly decreases to such low value that the MEMS capacitance is biased by a constant voltage instead of constant charge.
Apart from the fact that the transition from constant charge to constant voltage limits the output voltage swing of the MEMS, and thus the maximum sound pressure that can be handled without gross distortion, the constant voltage bias also causes the electrostatic pull-in at large diaphragm deflection.
The state-of-the-art solution for electrostatic pull-in is to integrate in the MEMS design some means that limit the mechanical deflection so that the electrostatic force in the constant voltage operation never exceeds the mechanical restoring force of the diaphragm. Depending of the mechanical design of the MEMS, there is a maximum stable bias voltage, which is well below the pull-in voltage, for stable constant voltage operation, which will form a practical limitation for the sensitivity of the microphone.
A solution for electrostatic pull-in is to ensure that under all conditions the MEMS will be biased in constant charge mode. If this can be fulfilled, there is no need for mechanical limitation of the diaphragm deflection, and the bias voltage can be increased to a level close to the pull-in voltage, thus allowing for higher microphone sensitivity.
A proposed solution (U.S. Pat. No. 8,630,429B2) employs a feedback network in order to control the behaviour of the high impedance. The disadvantage of this solution is that the dynamic range is still limited by the maximum voltage swing on the output of the feedback amplifier.
Other solutions may be seen in EP2648333, W02012/148077, EP2978241, US2010/246859 and US2013/051582.